1. Field of the Invention
The present invention relates to a method and an apparatus for driving a piezoelectric transformer used for a high-voltage power supply apparatus such as a discharge lamp starter for a liquid-crystal panel with backlighting, a copier and a page printer.
2. Description of the Related Art
A piezoelectric transformer is formed using a piezoelectric vibrator made up of a thin oblong sheet of piezoelectric material or a piezoelectric vibrator made by laminating a plurality of these thin oblong sheets. Piezoelectric materials used are barium titanate-based or lead zirconate titanate (PZT)-based piezoelectric ceramics, etc. The piezoelectric transformer consists of applying an AC (drive) voltage to an electrode (primary side) placed at one end of this piezoelectric vibrator in the thickness direction during a drive, exciting mechanical vibration and having another electrode (secondary side) at the other end of the piezoelectric vibrator convert this mechanical vibration to a voltage and outputting this voltage.
The above described piezoelectric transformer contains a resistance component, capacitance component, inductance component and configures a resonant circuit equivalently. Thus, it is conventionally believed that driving the piezoelectric transformer at a resonance frequency for this equivalent resonance circuit provides an efficient way of extracting electric power, that is, a high output voltage (power).
By the way, the resonance frequency of the aforementioned piezoelectric transformer also changes depending on conditions such as the input voltage level, load and temperature. Thus, to extract the performance of the piezoelectric transformer, it is necessary to change the drive frequency in accordance with changes in this resonance frequency. However, it is generally difficult to directly detect the resonance frequency of a circuit element in operation. Therefore, the drive frequency has been conventionally made to follow up changes in the resonance frequency by controlling the drive frequency so that the phase of the input voltage and the phase of the input current detected coincide or the difference between these two becomes constant.
However, the phase difference itself changes at the resonance frequency of the piezoelectric transformer depending on conditions such as the input voltage level, load and temperature. Thus, no matter how much the drive frequency is controlled so that the phase difference between the input voltage and input current of the piezoelectric transformer is constant, it is difficult to always extract power efficiently.
FIG. 1A to FIG. 1E illustrate examples of frequency characteristics of a piezoelectric transformer at an input voltage of 1 Vrms and load resistance of 100 kxcexa9 with respect to output power, output voltage, conversion efficiency (=output power/input power; ratio of output power to input power), input phase difference (phase difference between input voltage and input current) and input impedance. Furthermore, FIG. 2A to FIG. 2E illustrate examples of frequency characteristics with the load resistance changed to 500 kxcexa9 and FIG. 3A to FIG. 3E illustrate examples of frequency characteristics with the load resistance changed to 1 Mxcexa9. Hereafter, FIG. 1A to FIG. 1E will be collectively called FIG. 1, FIG. 2A to FIG. 2E will be collectively called FIG. 2 and FIG. 3A to FIG. 3E will be collectively called FIG. 3.
From FIG. 1 to FIG. 3, it is clear that when load resistance is changed, all the characteristics of output power, output voltage, conversion efficiency, input phase difference and input impedance change. Therefore, controlling so that the phase difference between the input voltage and input current of the piezoelectric transformer becomes constant will result in the output power, output voltage and conversion efficiency, etc. changed according to the load condition.
FIG. 4 illustrates the input phase difference corresponding to a maximum conversion efficiency under different load conditions (load resistance of 100 kxcexa9, 500 kxcexa9 and 1 Mxcexa9) when the input voltage is changed (1, 2, 3, 4 and 5 Vrms).
It is an object of the present invention to provide a method and apparatus for driving a piezoelectric transformer capable of driving with a maximum conversion efficiency and supplying a relatively high output voltage regardless of changes in the input voltage level, load and temperature, etc.
A detailed study of the conventional characteristic examples shown in FIG. 1 to FIG. 3 shows that the frequency corresponding to a maximum conversion efficiency and the frequency corresponding to a minimum input phase difference coincide regardless of the magnitude of load resistance. This may be attributable to the fact that when the input phase difference reaches a minimum value, the power factor approaches 1 and the apparent power on the input side decreases, which results in maximum efficiency.
Thus, the present invention has made it possible to always drive the piezoelectric transformer with maximum conversion efficiency by inputting an AC signal at a frequency corresponding to a minimum phase difference between the voltage and current on the input side (primary side) to the piezoelectric transformer, regardless of changes in the load, etc.
On the other hand, regarding the conventional characteristic examples shown in FIG. 1 to FIG. 3, the frequency corresponding to a maximum conversion efficiency and the frequency corresponding to a maximum output power (voltage) do not coincide, but the frequency corresponding to the maximum output power is lower. Thus, the present invention inputs an AC signal with a frequency lower than the frequency corresponding to the minimum phase difference (input phase difference) between the input voltage and input current by 0 to 30% of the difference between the resonance frequency and antiresonance frequency of the piezoelectric transformer. That is, the present invention has made it possible to drive the piezoelectric transformer at a frequency lower than the frequency corresponding to the maximum conversion efficiency by a certain frequency, for example, a frequency intermediate between the frequency corresponding to the maximum conversion efficiency and the frequency corresponding to the maximum output power regardless of changes in the load, etc. This allows the piezoelectric transformer to be driven with high conversion efficiency and large output power.
Such a driving method is implemented by a piezoelectric transformer drive apparatus provided with a unit that controls the frequency of an AC signal so that the phase difference between the input voltage and input current on the primary side of the piezoelectric transformer becomes a minimum, or by a piezoelectric transformer drive apparatus provided with a unit that controls the frequency of an AC signal so that it is lower than the frequency corresponding to the minimum phase difference by 0 to 30% of the difference between the resonance frequency and antiresonance frequency of the piezoelectric transformer.
Here, the unit that controls the frequency of the AC signal so that the phase difference between the input voltage and input current for the piezoelectric transformer becomes a minimum can be implemented by a unit that sweeps the frequency of the AC signal, a unit that detects a phase difference between the input voltage and input current on the primary side of the piezoelectric transformer, a unit that detects a minimum value of this phase difference and a unit that holds the frequency of the AC signal when this minimum value is obtained.
A more specific unit to control the frequency of the AC signal input to the piezoelectric transformer so that the phase difference reaches a minimum value can be implemented by a voltage control oscillator that generates an AC signal at a frequency according to the input voltage, a sweep circuit that generates a time-varying sweep voltage as the input voltage for this voltage control oscillator, a phase difference detector that detects a phase difference between the voltage and current on the primary side of the piezoelectric transformer and outputs a signal corresponding to this phase difference, a minimum value detection circuit that detects a minimum value of this phase difference signal and a sample-and-hold circuit that supplies the sweep voltage to the voltage control oscillator and holds the voltage when the minimum value of the phase difference signal is obtained and supplies it to the voltage control oscillator.
In this case, it is possible to reduce deviation of the conversion efficiency from its maximum value during a drive by sweeping the frequency of the AC signal at predetermined regular intervals or, in addition to this, when conditions of load and input, etc. are changed.
Furthermore, another unit that controls the frequency of an AC signal input to the piezoelectric transformer so that the phase difference reaches a minimum value can be implemented by a voltage control oscillator that generates an AC signal at a frequency according to the input voltage, a phase comparator that compares the phases of the voltage and current on the primary side of the piezoelectric transformer and supplies a voltage that holds a certain value when this phase difference reaches a minimum value under a predetermined load condition to the voltage control oscillator and a phase adjustment circuit that adjusts the phase of a signal input to the phase comparator above according to changes of the load condition, etc.
In this case, it is possible to improve the conversion efficiency during adjustment of power to the load, by controlling the amount of phase adjustment by the phase adjustment circuit according to the amount of control of adjusting power supplied to the load.